updating to version 12.0.0
took a look into upstream's Gopkg.lock, checking for modifications;
changed 2 depencies to use version instead of revision;
google:go-cmp and golang:text
get QA done right; thanks to portlint;
moved PLIST_FILES to appear after USES.
Approved by: araujo (mentor), rene (mentor)
Differential Revision: https://reviews.freebsd.org/D17530